Acid-Base Equilibria and Buffer Solutions Video Lecture | Physical Chemistry

professor Dave here, let's discuss
acid-base equilibria
when a strong acid or base reacts with water we can say
that it completely ionizes, that
every molecule of acid will transfer its
acidic proton to a water molecule. if
every molecule reacts to completion this
makes things neat and tidy but when weak
acids and bases react they only partially
ionize and they establish an
equilibrium between the ionized and
unionzed form and that means we have
to do some math. Ka is the acid
ionization constant. this is derived just
like we got Kc for generic chemical
equilibria. note that we don't include
water as it is a pure liquid. in this way
Ka is a measure of how thoroughly an
acid-base reaction proceeds in the
forward direction. a larger Ka means more
products and a stronger acid a smaller
Ka means more reactants and a weaker
acid so an acid with a larger Ka will be
more acidic and therefore associated
with a lower pH. we can determine Ka
experimentally. let's say we want to know
the Ka for a weak acid like phenol. we can
set up an icebox just like the kind we
already know. we can use the pH to
calculate hydronium concentration
and therefore also the concentration of
the conjugate base since they are the
same
as it happens the change in initial
concentration of the acid is so small
that it's negligible and we treat things
as though none got used up
plug these values into the expression
and solve for K
from this information we can also
calculate the degree of ionization which
is the ratio of ionized to unionized
acid molecules in solution or the
percent ionization which is the same
thing but expressed as a percentage. this
is a pretty weak acid considering so few
molecules transferred a proton to a
water molecule
sometimes like in the previous example
the acid is so weak that we pretend the
concentration of the acid doesn't change
but if it's a little stronger than that
we can't make that simplifying
assumption and the math gets trickier
but still essentially the same. we can
make ice boxes for weak bases as well, or
multiple ice boxes for polyprotic
acids, one per ionization, just more math
a buffer is a solution that has the
ability to resist changes in pH
when limited amounts of acid or base are
added to it
buffers are made by combining a weak
acid or base with its conjugate. in such
a solution because both the acid and
conjugate base are present they are able
to neutralize small amounts of any acid
or base that are added to the solution
many biological fluids are buffer
solutions because they must maintain a
steady pH to avoid loss of critical
biological function. the bloodstream for
example maintains a pH of around 7.4
otherwise its capacity to carry oxygen
around the body would be lost
buffer solutions of a particular pH can
be prepared using the Henderson-Hasselbalch
equation. we know about pH and pKa
is similar in that if pH is the negative
log of the hydronium concentration, pKa
is the negative log of the Ka for that
acid. what this shows us is that in
order to create a buffer solution of a
desired pH we must use an acid conjugate-base pair where the Ka is approximately
equal to the desired hydronium
concentration or rather where the pKa is
equal to the pH. let's check
comprehension
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